U.S. patent application number 10/647347 was filed with the patent office on 2004-03-11 for inductive plasma processor method.
This patent application is currently assigned to LAM Research Corporation. Invention is credited to Chen, Jian J., Veltrop, Robert G., Wicker, Thomas E..
Application Number | 20040045506 10/647347 |
Document ID | / |
Family ID | 25232318 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040045506 |
Kind Code |
A1 |
Chen, Jian J. ; et
al. |
March 11, 2004 |
Inductive plasma processor method
Abstract
An inductive plasma processor includes a multiple winding radio
frequency coil having plural electrically parallel, spatially
concentric windings (1) having different amounts of RF power
supplied to them, and (2) arranged to produce electromagnetic
fields having different couplings to different regions of plasma in
the chamber to control plasma flux distribution incident on a
processed workpiece. The coil is powered by a single radio
frequency generator via a single matching network. Input and output
ends of each winding are respectively connected to input and output
tuning capacitors. In a first embodiment, the location of maximum
inductive coupling of the radio frequency to the plasma and the
current magnitude in each winding are respectively mainly
determined by values of the output and input capacitors. By
adjusting all the input and output capacitors simultaneously, the
current to a winding can be varied while the current to the other
winding can be maintained constant as if these windings were
completely de-coupled and independent. Therefore, the capacitors
can control the plasma density in different radial and azimuthal
regions. In another embodiment, a relatively low frequency drives
the coil whereby each winding has a relatively short electrical
length, causing substantially small standing wave current and
voltage variations. The output capacitor for each winding adjusts
current magnitude, to eliminate the need for the input capacitors
and reduce operational complexity.
Inventors: |
Chen, Jian J.; (Fremont,
CA) ; Veltrop, Robert G.; (Eagle, ID) ;
Wicker, Thomas E.; (Reno, NV) |
Correspondence
Address: |
LOWE HAUPTMAN GILMAN & BERNER, LLP
Suite 300
1700 Diagonal Road
Alexandria
VA
22314
US
|
Assignee: |
LAM Research Corporation
|
Family ID: |
25232318 |
Appl. No.: |
10/647347 |
Filed: |
August 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10647347 |
Aug 26, 2003 |
|
|
|
09821027 |
Mar 30, 2001 |
|
|
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Current U.S.
Class: |
118/723I ;
156/345.48 |
Current CPC
Class: |
H01J 37/32174 20130101;
H01J 37/321 20130101 |
Class at
Publication: |
118/723.00I ;
156/345.48 |
International
Class: |
H01L 021/306 |
Claims
What is claimed is:
1. A method of controlling the distribution of electromagnetic
fields launched by an excitation coil to excite a plasma in a
vacuum plasma processor for processing a workpiece, the coil
including plural parallel windings for coupling electromagnetic
fields to plasma in the chamber, the method comprising controlling
(a) the total amount of power applied to the plural windings so
that for different distributions of electromagnetic fields
different amounts of total power are applied to the plural
windings, and (b) the amount of current applied to individual
plural windings so that for different distributions of
electromagnetic fields different amounts of current are applied to
the individual windings.
2. The method of claim 1 wherein the windings are arranged so (a)
one of the windings is an exterior winding located so
electromagnetic fields generated by it are in proximity to a
peripheral wall of the chamber, and (b) electromagnetic fields
generated by the remainder of the coil are remote from the chamber
peripheral wall, and controlling the current applied to the
exterior winding so the electromagnetic fields generated by the
exterior winding exceeds the electromagnetic fields generated by
the remainder of the coil.
3. The method of claim 1 wherein the windings are arranged so (a)
one of the windings is an exterior winding located so
electromagnetic fields generated by it are in proximity to a
peripheral wall of the chamber, and (b) electromagnetic fields
generated by the remainder of the coil are remote from the chamber
peripheral wall, and controlling the current applied to the
exterior winding so the electromagnetic field generated by the
exterior winding is less than the electromagnetic field generated
by the remainder of the coil.
4. The method of claim 1 wherein the windings are arranged so (a)
one of the windings is an exterior winding located so
electromagnetic fields generated by it are in proximity to a
peripheral wall of the chamber, and (b) electromagnetic fields
generated by the remainder of the coil are remote from the chamber
peripheral wall, and controlling the current applied to the
exterior winding so the electromagnetic field generated by the
exterior winding is about the same as the electromagnetic field
generated by the remainder of the coil.
5. The method of claim 1 wherein each winding includes first and
second terminals, the first terminal being connected via a first
series capacitor to an output terminal of a matching network driven
by a source of the power, the second terminal being connected via a
second series capacitor to a ground terminal, the controlling steps
for the current in the individual windings being performed by
controlling the value of at least one capacitor associated with
each individual winding and the total power in the windings.
6. The method of claim 1 further including maintaining the power
coupled to one of the windings substantially constant for different
distributions and changing the power coupled to another of the
windings for the different distributions.
7. The method of claim 6 wherein the maintaining and changing steps
are performed by controlling the values of impedances associated
with the individual windings and the total power applied to the
coil.
8. The method of claim 7 wherein each winding includes first and
second terminals, the first terminal being connected via a first
series capacitor to an output terminal of a matching network driven
by a source of the power, the second terminal being connected via a
second series capacitor to a ground terminal, and controlling the
values of the impedances by controlling the values of at least one
capacitor associated with each individual winding.
9. The method of claim 8 wherein the effects due to substantial
standing wave current variations at a relatively high RF frequency
along the lengths of the individual windings are minimized by
adjusting the value of at least one capacitor associated with each
winding so that adjacent windings have standing wave RF current
maxima that are radially opposite to each other.
10. The method of claim 1 wherein the power is RF having a
frequency and the windings have lengths such that there are no
substantial standing wave current variations along the lengths of
the individual windings, and adjusting the value of an impedance
coupled with each winding so that there is substantially uniform
plasma density distribution on the workpiece.
11. An inductive plasma processor for processing a workpiece,
comprising a plasma excitation coil, the coil including plural
parallel windings, a source for supplying power to the plural
parallel windings, variable impedance arrangements respectively
coupled with the parallel windings for controlling the currents
flowing from the source to each of the windings, and a controller
for controlling the total power the source supplies to the parallel
windings and components of the variable impedance arrangements.
12. The processor of claim 11 wherein the controller is arranged
for controlling the total power and the variable impedance
arrangements so that for different distributions of electromagnetic
fields generated by and supplied by the different windings to the
plasma the current flowing in one of the windings remains
substantially constant and the current in the remainder of the coil
changes.
13. The processor of claim 12 wherein each of the impedance
arrangements includes a variable reactance coupled to its
respective winding, the variable reactance of each impedance
arrangement being arranged for controlling the location of the
maximum amplitude of a standing wave current in its respective
winding, the controller being arranged for controlling the values
of the variable reactance of each impedance arrangement.
14. The processor of claim 13 wherein the source is an RF source,
the frequency of the RF source and the length of the windings are
such that there are substantial standing wave current variations
along the length of each winding.
15. The processor of claim 12 wherein each of the impedance
arrangements includes a variable reactance coupled to its
respective winding, the variable reactance of each impedance
arrangement being arranged for controlling the value of the maximum
amplitude of a standing wave RF current in its respective winding,
the controller being arranged for controlling the value of the
variable reactance of each impedance arrangement.
16. The processor of claim 15 wherein the source is an RF source,
the frequency of the RF source and the length of the windings being
such that there are no substantial standing wave current variations
along the length of each winding.
17. The processor of claim 12 wherein the source is an RF source,
each of the windings including first and second end terminals and
each of the impedance arrangements includes first and second
variable capacitors, each of the first capacitors being connected
in series with its respective first terminal for supplying RF
energy from the RF source to the respective winding, each of the
second capacitors being connected in series between its respective
second terminal and ground, the controller being arranged for
controlling the values of the first and second variable
capacitors.
18. The processor of claim 17 wherein the first and second
capacitors are arranged so their values control the magnitude and
location of the maximum amplitude of a standing wave RF current in
their respective winding.
19. The processor of claim 12 wherein the source is an RF source,
the frequency of the RF source and the length of the windings being
such that there are no substantial standing wave current variations
along the length of each winding, and each variable impedance
arrangement includes a single variable reactance coupled with each
winding, the controller being arranged for controlling the value of
the variable reactance to control the maximum amplitude of the
standing wave current in each winding.
20. An inductive plasma processor for processing a workpiece,
comprising a plasma excitation coil, the coil including plural
parallel windings, a source for supplying power to the plural
parallel windings, impedance arrangements respectively coupled with
the parallel windings, the power of the source and the values of
reactances of the impedance arrangements being such that (a) the
maximum amplitude of a standing wave current in one of the windings
differs from the maximum amplitude of a standing wave current in
the remainder of the coil and (b) adjacent windings have standing
wave current maxima that are radially opposite to each other.
21. The processor of claim 20 wherein each of the windings is
arranged for coupling an electromagnetic field to plasma in the
chamber, one of the windings being an exterior winding located so
electromagnetic fields generated by it is in proximity to a
peripheral wall of the chamber, the remainder of the coil being
arranged so electromagnetic fields generated by the remainder of
the coil are remote from the chamber peripheral wall, the values of
the total power the source supplies to the coil and of the
reactances being such that the electromagnetic field generated by
the exterior winding exceeds the electromagnetic field generated by
the remainder of the coil.
22. The processor of claim 20 wherein each of the windings is
arranged for coupling an electromagnetic field to plasma in the
chamber, one of the windings being an exterior windings located so
an electromagnetic field generated by it is in proximity to a
peripheral wall of the chamber, the remainder of the coil being
arranged so electromagnetic fields generated by the remainder of
the coil are remote from the chamber peripheral wall, the values of
the total power the source supplies to the coil and the reactances
being such that the electromagnetic field generated by the exterior
winding is less than the electromagnetic field generated by the
remainder of the coil.
23. The processor of claim 20 wherein each of the windings is
arranged for coupling an electromagnetic field to plasma in the
chamber, one of the windings being an exterior winding located so
an electromagnetic field generated by it is in proximity to a
peripheral wall of the chamber, the remainder of the coil being
arranged so electromagnetic fields generated by the remainder of
the coil are remote from the chamber peripheral wall, the values of
the total power the source supplies to the coil and of the
reactances being such that the electromagnetic field generated by
the exterior winding is about the same as the electromagnetic field
generated by the remainder of the coil.
24. The processor of claim 19 wherein the source is an RF source,
the RF source frequency and the lengths of the windings are such
that there are no substantial standing wave current variations
along the length of each winding, the reactance coupled with each
winding being arranged for controlling the value of the standing
wave current in the respective winding.
25. An inductive plasma processor for processing a workpiece,
comprising a plasma excitation coil, the coil including plural
parallel windings, a source for supplying power to the plural
parallel windings, impedance arrangements respectively coupled with
the parallel windings, the source frequency and the lengths of the
windings being such that there are no substantial standing wave
current variations along the length of each winding, the impedance
arrangement coupled with each winding being arranged for
controlling the value of the standing wave current in the
respective winding.
26. A method of controlling the plasma flux distribution on a
workpiece of an inductive plasma processor including a plasma
excitation coil having a center axis and plural parallel windings
adapted to be driven by an excitation source, the plural windings
being concentric with the axis so an exterior winding of the coil
surrounds the remainder of the coil, the method comprising
positioning the exterior winding relative to the remainder of the
coil so the plasma density incident on the workpiece has a
predetermined desired relationship.
27. The method of claim 26 wherein the positioning step includes
turning the exterior winding and another winding of the coil
relative to each other about the axis.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to inductive
plasma.vertline.workpiece processors including an RF plasma
excitation coil and, more particularly, to such processors wherein
the coil includes plural windings (1)having different amounts of RF
power supplied to them, and (2)arranged to supply RF magnetic
fields having different flux magnitudes to plasma in the chamber to
control plasma flux distribution incident on a processed workpiece,
and to a method of operating same. The invention also relates to an
inductive plasma workpiece processor including a coil having plural
parallel electrically short windings connected to impedance
arrangements for controlling the amplitude of current flowing in
the windings. The invention also relates to a method of making an
inductive plasma processor wherein one winding of a coil is
positioned relative to another winding of the coil for optimum
workpiece processing.
BACKGROUND ART
[0002] An inductive plasma processor treats workpieces with an RF
plasma in a vacuum chamber and includes a coil responsive to an RF
source. The coil, which can be planar or spherical or dome shaped,
is driven by the RF source to generate electromagnetic fields that
excite ionizable gas in the chamber to produce a plasma. Usually
the coil is on or adjacent to a dielectric window that extends in a
direction generally parallel to a planar horizontally extending
surface of the processed workpiece. The excited plasma interacts
with the workpiece in the chamber to etch the workpiece or to
deposit material on it. The workpiece is typically a semiconductor
wafer having a planar circular surface or a solid dielectric plate,
e.g., a rectangular glass substrate used in flat panel displays, or
a metal plate.
[0003] Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral
planar coil for achieving the above results. The spiral, which is
generally of the Archimedes type, extends radially and
circumferentially between its interior and exterior terminals
connected to the RF source via an impedance matching network. Coils
produce oscillating RF fields having magnetic and electric field
components that penetrate through the dielectric window to excite
electrons and ions in a portion of the plasma chamber close to the
window. The spatial distribution of the magnetic field in the
plasma portion close to the window is a function of the sum of
individual magnetic field components produced by the current at
each point of the coils. The inductive component of the electric
field is produced by the time varying magnetic field, while the
capacitive component of the electric field is produced by the RF
voltage in the coils. The inductive electric field is azimuthal
while the capacitive electric field is vertical to the workpiece.
The current and voltage differ at different points because of
transmission line effects of the coil at the frequency of the RF
source.
[0004] For spiral designs as disclosed by and based on the Ogle
'458 patent, the RF currents in the spiral coil are distributed to
produce a toroidal shaped electric field resulting in a toroidal
plasma close to the window, which is where power is absorbed by the
gas to excite the gas to a plasma. The toroidal shaped magnetic
field is accompanied by a ring shaped electric field which
generates a toroidal shaped plasma distribution. At low pressures,
in the 1.0 to 10 mTorr range, diffusion of the plasma from the
toroidal shaped region where plasma density is peaked tends to
smear out plasma non-uniformity and increases plasma density in the
chamber center just above the center of the workpiece. However, the
diffusion alone generally can not sufficiently compensate plasma
losses to the chamber walls and plasma density around the workpiece
periphery can not be changed independently. At intermediate
pressure ranges, in the 10 to 100 mTorr range, gas phase collisions
of electrons, ions, and neutrals in the plasma further prevent
substantial diffusion of the plasma charged particles from the
toroidal region. As a result, there is a relatively high plasma
density in a ring like region of the workpiece but low plasma
densities in the center and peripheral workpiece portions.
[0005] These different operating conditions result in substantially
large plasma flux (i.e., plasma density) variations between inside
the toroid and outside the toroid, as well as at different
azimuthal angles with respect to a center line of the chamber that
is at right angles to the plane of the workpiece holder (i.e.,
chamber axis). These plasma flux variations result in a substantial
standard deviation, i.e., in excess of six percent, of the plasma
flux incident on the workpiece. The substantial standard deviation
of the plasma flux incident on the workpiece has a tendency to
cause non-uniform workpiece processing, i.e, different portions of
the workpiece are etched to different extents and/or have different
amounts of materials deposited on them.
[0006] Many coils have been designed to improve the uniformity of
the plasma. The commonly assigned U.S. Pat. No. 5,759,280, Holland
et al., issued Jun. 2, 1998, discloses a coil which, in the
commercial embodiment, has a diameter of 12 inches and is operated
in conjunction with a vacuum chamber having a 14.0 inch inner wall
circular diameter. The coil applies magnetic and electric fields to
the chamber interior via a quartz window having a 14.7 inch
diameter and 0.8 inch uniform thickness. Circular semiconductor
wafer workpieces are positioned on a workpiece holder about 4.7
inches below a bottom face of the window so the center of each
workpiece is coincident with a center line of the coil and the
chamber center line.
[0007] The coil of the '280 patent produces considerably smaller
plasma flux variations across the workpiece than the coil of the
'458 patent. The standard deviation of the plasma flux produced by
the coil of the '280 patent on a 200 mm wafer in such a chamber
operating at 5 milliTorr is a considerable improvement over the
standard deviation for a coil of the '458 patent operating under
the same conditions. The coil of the '280 patent causes the
magnetic field to be such that the plasma density in the center of
the workpiece is greater than in an intermediate part of the
workpiece, which in turn exceeds the plasma density in the
periphery of the workpiece. The plasma density variations in the
different portions of the chamber for the coil of the '280 patent
are much smaller than those of the coil of the '458 patent for the
same operating conditions as produce the lower standard
deviation.
[0008] Other arrangements directed to improving the uniformity of
the plasma density incident on a workpiece have also concentrated
on geometric principles, usually concerning coil geometry. See,
e.g., U.S. Pat. Nos. 5,304,279; 5,277,751; 5,226,967; 5,368,710;
5,800,619; 5,401,350; 5,558,722, 5,795,429, 5,847,074 and
6,028,395. However, these coils have generally been designed to
provide improved radial plasma flux uniformity and to a large
extent have ignored azimuthal plasma flux uniformity. In addition,
the fixed geometry of these coils does not permit the plasma flux
distribution to be changed for different processing recipes. While
we are aware that the commonly assigned co-pending U.S. application
of John Holland for "Plasma Processor with Coil Responsive to
Variable Amplitude RF Envelope," Ser. No. 09/343,246, filed Jun.
30, 1999, and Gates U.S. Pat. No. 5,731,565 disclose electronic
arrangements for at will controlling plasma flux uniformity for
different recipes, the Holland and Gates inventions are concerned
primarily with radial, rather than azimuthal, plasma flux
uniformity. In the Holland invention, control of the plasma flux
uniformity is achieved by controlling a variable amplitude envelope
the RF excitation source applies to the coil. In the Gates
invention, a switch or a capacitor shunts an interior portion of a
spiral-like RF plasma excitation coil.
[0009] The frequency, i.e., reciprocal of wavelength, (typically
13.56 MHz) of the RF power source driving the coil and the lengths
of the coil are such that there are significant standing wave
current and voltage variations along the length of a particular
winding. Voltage magnitude can change from about 1,000 volts (rms)
to nearly zero volts, while the standing wave current can change
nearly 50%. Hence, there are peak voltage and current somewhere
along the length of each winding. However, we are aware of prior
art including an RF source that drives an electrically short plasma
excitation coil.
[0010] Our U.S. Pat. No. 6,164,241, entitled "Multiple Coil Antenna
for Inductively-Coupled Plasma Generation Systems," discloses
another coil including two concentric electrically parallel
windings each having first and second terminals, which can be
considered input and output terminals of each winding. Each first
terminal is connected via a first series capacitor to an output
terminal of a matching network driven by an RF power source. Each
second terminal is connected via a second series capacitor to a
common ground terminal of the matching network and RF source. Each
winding can include a single turn or multiple turns that extend
circumferentially and radially in a spiral-like manner relative to
a common axis of the two windings. Each winding is planar or
three-dimensional (i.e., spherical or dome-shaped) or separate
turns of a single winding can be stacked relative to each other to
augment the amount of magnetic flux coupled by a particular winding
to the plasma.
[0011] The value of the second capacitor connected between the
second terminal of each winding and ground sets the locations of
the voltage and current extrema (i.e., maximum and minimum) in each
winding, as disclosed in Holland et al., U.S. Pat. No. 5,759,280,
commonly assigned with the present invention. Controlling the value
of the second capacitor of each winding controls the distribution
of magnetic flux produced by the coil to the plasma and the plasma
flux incident on the workpiece because the value of the capacitor
determines the location of the maximum values of the RF standing
wave current and voltage in each respective winding. The value of
the first capacitor determines the maximum magnitude of the current
and voltage standing waves in each winding. The values of the first
capacitors are also adjusted to help maintain a tuned condition
between the RF source and the load it drives, which is primarily
the coil and the plasma load coupled to the coil. Adjusting the
maximum magnitude and location of the standing wave current in each
winding controls the plasma density in different radial and
azimuthal regions of the chamber.
[0012] It is desirable, in certain instances, to maintain the
current in one of the windings relatively constant while changing
the current in the remainder of the coil. The RF current generates
the magnetic field, and the time varying magnetic field in free
space produces the inductive electric field, which in turn
generates the plasma and induces a plasma "image" current which is
the mirror image of the driving RF current. By maintaining the
current in one of the windings relatively constant, the electric
field produced by that winding and supplied to the plasma in the
chamber remains relatively constant, despite variations in the
electric field produced by the remainder of the coil and supplied
to the plasma. Maintaining the electric field produced by one of
the windings relatively constant while varying the electric field
produced by the remainder of the coil and supplied to the plasma
provides substantial control for the plasma density incident on the
workpiece. Such control is particularly advantageous in connection
with processing chambers operating with different recipes, which
are performed without opening the vacuum chamber. Such chambers
operate at different times under differing conditions. Examples of
the different conditions are different processing gases, different
pressures and different workpieces.
[0013] Consider a coil having first and second parallel, concentric
windings respectively close to (1) the chamber periphery and (2)
the chamber axis. The first and second windings respectively couple
ring shaped electric fields to the peripheral portions of the
chamber (close to the chamber wall) and to the chamber center. It
is desirable, in certain instances, to maintain the current flowing
in the outer winding substantially constant at times, while
differing currents flow in the inner windings. This causes the
outer winding to produce a substantially constant electric field in
the chamber peripheral portions while the inner winding generates
different electric fields in the chamber central region. Such a
result is attained by simultaneously adjusting the overall
impedance in each winding and the total power since these windings
are closely coupled to both windings. Since these windings are
closely coupled, the change of the overall impedance in each
winding causes change in current splitting as well as power
splitting between these windings. The current in each winding is
changed as the impedance in any winding changes. Therefore, the
current in one winding can be compensated by changing the total
power in order to maintain constant current in that winding. The
ability to maintain a constant electric field in the chamber
peripheral portion results in an extra process control knob to
maintain constant power deposition in that region and further
maintain constant processing results (e.g. etch rate or deposition
rate) on the peripheral portion of a workpiece. This process
control is particularly useful to compensate changes due to process
conditions. In other situations, particularly for other pressures,
as discussed supra, it is desirable to maintain the electric field
in the chamber center substantially constant at times while the
amplitude of the electric field in a peripheral portion of the
chamber is changed. This process control capability is particularly
useful to compensate the plasma loss to chamber walls and electric
coupling to the grounded portion of chamber walls.
[0014] It is accordingly an object of the present invention to
provide a new and improved vacuum plasma processor and method of
operating same wherein the plasma density incident on the workpiece
can be controlled at will.
[0015] An additional object of the present invention is to provide
a new and improved vacuum plasma processor and method of operating
same wherein the plasma density incident on a workpiece has
relatively high uniformity.
[0016] An additional object of the present invention to provide a
new and improved vacuum plasma processor and method of operating
same wherein plasma density incident on a workpiece of the
processor has relatively high azimuthal uniformity.
[0017] A further object of the invention is to provide a new and
improved vacuum plasma processor including a coil with plural
parallel windings driven by a single RF source via a single
matching network and having improved control for the electric and
magnetic fields that are produced by the coil and coupled to plasma
in the chamber.
SUMMARY OF THE INVENTION
[0018] In accordance with one aspect of the invention, there is an
improved control method and apparatus for the distribution of
electromagnetic fields coupled by a plasma excitation coil to a
plasma in a vacuum plasma processor for processing a workpiece,
wherein the coil includes plural parallel windings for coupling
inductive electric fields to plasma in the chamber. In the method
and apparatus, (a) the total amount of RF power applied to the
plural windings is controlled so that for different distributions
of electromagnetic fields, different amounts of total RF power are
applied to the plural windings and (b) the amount of RF current
applied to individual plural windings is controlled so that for
different distributions of the electromagnetic fields, different
amounts of RF current are applied to the individual windings.
[0019] In a preferred embodiment, the windings are arranged so (a)
one of the windings is an exterior winding located so the
electromagnetic fields generated by it are in proximity to a
peripheral wall of the chamber, and (b) electromagnetic fields
generated by the remainder of the coil are remote from the chamber
peripheral wall. The RF current applied to the exterior winding is
controlled so the electromagnetic fields generated by the exterior
winding exceeds the electromagnetic fields generated by the
remainder of the coil in one arrangement. In a second arrangement,
the electromagnetic fields generated by the exterior winding is
less than electromagnetic fields generated by the remainder of the
coil. In another arrangement, which results in a very nearly
uniform plasma density on the workpiece, the RF currents applied to
the exterior winding and the remainder of the coil are somewhat
equal.
[0020] By controlling the currents, the RF power coupled to one of
the windings is maintained substantially constant for different
electromagnetic fields distributions and the RF power coupled to
another of the windings is changed for the different distributions.
The power maintaining and changing operations are preferably
performed by controlling the values of impedances associated with
the individual windings and the total power applied to the coil. In
one embodiment, each of the plural windings includes first and
second terminals such that the first terminal is connected via a
first series capacitor to an output terminal of a matching network
driven by a source of the RF power and the second terminal is
connected via a second series capacitor to the common ground
terminal of the matching network. The values of the impedances are
controlled by controlling the capacitance of at least one capacitor
associated with each individual winding.
[0021] In one embodiment, the RF power has a frequency and the
windings have lengths such that there are substantial standing wave
current variations along the lengths of the individual windings. In
such a configuration, the value of at least one capacitor
associated with each winding is adjusted so that adjacent windings
have standing wave RF current maxima that are radially opposite to
each other.
[0022] In another aspect of the invention, the RF power has a
frequency and the windings have lengths such that there are no
substantial standing wave current variations along the lengths of
the individual windings. In such a configuration, only one
capacitor need be associated with each winding and its capacitance
is adjusted to control the current amplitude flowing in the
winding.
[0023] A further aspect of the invention involves positioning an
exterior winding of the coil relative to the remainder of the coil
to achieve substantially uniform plasma density on the workpiece.
In particular, the exterior winding is turned about an axis of the
coil, relative to the remainder of the coil which is substantially
concentric with and inside the exterior winding.
[0024] The above and still further objects, features and advantages
of the present invention will become apparent upon consideration of
the following detailed descriptions of several specific embodiments
thereof, especially when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWING
[0025] FIG. 1 is a schematic diagram of a vacuum plasma processor
of the type employed in connection with the present invention;
[0026] FIG. 2 is an electrical schematic diagram of a coil included
in the processor of FIG. 1 in combination with an RF source, a
matching network, and electronic control circuitry for driving the
coil and for controlling (1) the capacitances of variable
capacitors connected to the coil and (2) the total power supplied
to the coil;
[0027] FIG. 3 includes calculated amplitudes of standing wave
currents in the windings of the coil illustrated in FIG. 2 for
excitations at 13.56 MHz and 4.0 MHz;
[0028] FIG. 4 is a circuit diagram of the matching network with
current sensors for driving the coil of FIG. 2; and
[0029] FIG. 5 is a sputter rate contour plot resulting from use of
the apparatus of FIGS. 1, 2 and 4.
DETAILED DESCRIPTION OF THE DRAWING
[0030] The vacuum plasma workpiece processor of FIG. 1 of the
drawing includes vacuum chamber 10, shaped as a cylinder having
grounded metal wall 12, metal bottom end plate 14, and circular top
plate structure 18, consisting of a dielectric window structure 19,
having the same thickness from its center to its periphery. Sealing
of vacuum chamber 10 is provided by conventional gaskets (not
shown). The processor of FIG. 1 can be used for etching a
semiconductor, dielectric or metal substrate or for depositing
materials on such substrates.
[0031] A suitable gas that can be excited to a plasma state is
supplied to the interior of chamber 10 from a gas source (not
shown) via port 20 in side wall 12 and further distributed
uniformly through a gas distribution mechanism (not shown). The
interior of the chamber is maintained in a vacuum condition, at a
pressure that can vary in the range of 1-1000 milliTorr, by a
vacuum pump (not shown) connected to port 22 in end plate 14.
[0032] The gas in chamber 10 is excited by a suitable electric
source to a plasma having a controlled spatial density. The
electric source includes a planar or spherical or dome like coil
24, mounted immediately above window 19 and excited by variable
power RF generator 26, typically having a fixed frequency of 13.56
MHz.
[0033] Impedance matching network 28, connected between output
terminals of RF generator 26 and excitation terminals of coil 24,
couples RF power from the generator to the coil. Impedance matching
network 28 includes variable reactances which controller 29 varies
in a known manner in response to indications of the amplitude and
phase angle of the voltage reflected to the input terminals of the
matching network, as sensed by detector 43. Controller 29 varies
the values of the reactances in network 28 to achieve impedance
matching between source 26 and a load including coil 24 and the
plasma load the coil drives.
[0034] Controller 29 also responds to input device 41 to control
variable reactances coupled to coil 24. Input device 41 can be a
manual device, such as a potentiometer or keys of a key pad, or a
microprocessor responsive to signals stored in a computer memory
for different processing recipes of workpiece 32. Variables of the
recipes include (1) species of gases flowing through port 20 into
chamber 10, (2) pressure in chamber 10 controlled by the vacuum
pump connected to port 22, (3) the total output power of RF source
26, which is substantially equal to the power, supplied to coil 24,
and (4) the values of capacitors connected to coil 24.
[0035] Workpiece 32 is fixedly mounted in chamber 10 to a surface
of workpiece holder (i.e., platen or chuck) 30; the surface of
holder 30 carrying workpiece 32 is parallel to the surface of
window 19. Workpiece 32 is usually electrostatically clamped to the
surface of holder 30 by a DC potential that a DC power supply (not
shown) applies to a chuck electrode (not shown) of holder 30. RF
source 45 supplies the radio frequency electromagnetic wave to
impedance matching network 47, that includes variable reactances
(not shown). Matching network 47 couples the output of source 45 to
holder 30. Controller 29 responds to signals that amplitude and
phase detector 49 derives to control the variable reactances of
matching network 47 to match the impedance of source 45 to the
impedance of an electrode (not shown) of holder 30. The load
coupled to the electrode in holder 30 is primarily the plasma in
chamber 10. As is well known the RF voltage source 45 applies to
the electrode of holder 30 interacts with charge particles in the
plasma to produce a DC bias on workpiece 32.
[0036] Surrounding coil 24 and extending above top end plate 18 is
a metal tube or can-like shield 34 having an inner diameter
somewhat greater than the inner diameter of wall 12. Shield 34
decouples electromagnetic fields originating in coil 24 from the
surrounding environment. The diameter of cylindrically shaped
chamber 10 defines the boundary for the electromagnetic fields
generated by coil 24. The diameter of dielectric window structure
19 is greater than the diameter of chamber 10 to such an extent
that the entire upper surface of chamber 10 is comprised of
dielectric window structure 19.
[0037] The distance between the treated surface of workpiece 32 and
the bottom surface of dielectric window structure 19 is chosen to
provide the most uniform plasma flux on the exposed, processed
surface of the workpiece. For a preferred embodiment of the
invention, the distance between the workpiece processed surface and
the bottom of the dielectric window is approximately 0.2 to 0.4
times the diameter of chamber 10.
[0038] Coil 24 includes plural parallel windings each of which is
electrically long enough at the 13.56 MHz frequency of source 26 to
function as a transmission line having a total electric length of
about 30 to 45.degree. to produce standing wave patterns along the
length of the winding. The standing wave patterns result in
variations in the magnitude of standing wave RF voltages and
currents along the lengths of the windings. The dependence of the
magnetic fluxes generated by the windings on the magnitude of these
RF currents results in different plasma density being produced in
different portions of chamber 10 beneath different windings of coil
24.
[0039] The variations in the RF current magnitude flowing in
different windings of the coil are spatially averaged to assist in
controlling plasma density spatial distribution. Spatially
averaging these different current values in the different windings
of the coil can substantially prevent azimuthal asymmetries in the
plasma density, particularly at regions of low RF current in the
windings. Alternatively, the frequency of generator 26 is 4.0 MHz,
in which case the windings of coil 24 are electrically short, about
10.degree. to 15.degree., causing the standing wave currents and
voltages in the windings to be substantially constant.
[0040] Controller 29 includes microprocessor 33 (FIG. 2) which
responds to (1) input control 41, (2) voltage amplitude and phase
angle signals that detector 31 derives, and (3) memory system 35
that stores programs for controlling microprocessor 33 as well as
signals controlling the values of variable capacitors connected to
coil 24 and the output power of RF generator 26. Among the programs
memory system 35 stores are control programs for the values of the
variable reactances of matching networks 28 and 47. The output
power of source 26 and the values of capacitors connected to coil
24 can also be pre-set at the time the processor is made or
installed, particularly if the processor is dedicated to a single
recipe.
[0041] As illustrated in FIG. 2, coil 24 includes two parallel
windings 40 and 42, both of which are generally concentric with
center coil axis 44 and include multiple spiral-like turns that
extend radially and circumferentially with respect to axis 44.
Interior winding 40 is wholly within exterior winding 42, such that
winding 42 completely surrounds winding 40. Winding 40 includes
interior terminal 46 and exterior terminal 48, while winding 42
includes exterior terminal 50 and interior terminal 52.
[0042] Interior winding 40 includes three concentric turns 54, 56
and 58 having different radii, as well as two straight segments 60
and 62. Each of turns 54, 56 and 58 is a segment of a circle
centered on axis 44 and having an angular extent of about
340.degree.. Adjacent ends of turns 54 and 56 are connected to each
other by straight segment 60, while straight segment 62
interconnects adjacent ends of turns 56 and 58 to each other.
Straight segments 60 and 62 extend radially and circumferentially
along substantially parallel paths.
[0043] Exterior winding 42 includes two concentric turns 64 and 66
having different radii, as well as straight segment 68. Each of
turns 64 and 66 is a segment of a circle centered on axis 44 and
having an angular extent of about 340.degree.. Straight segment 68
extends radially and circumferentially to connect adjacent ends of
turns 64 and 66 to each other.
[0044] The sum of the lengths of turns 54, 56 and 58 and sectors 60
and 62 of winding 40 is about equal to the sum of the lengths of
turns 64 and 66, as well as sector 68 of winding 42. Because
windings 40 and 42 have substantially equal lengths, they have
standing wave voltage and current variations along their length
which are substantially the same, regardless of the frequency that
generator 26 supplies to them.
[0045] Windings 40 and 42 of coil 24 are driven in parallel by RF
current derived by a single fixed frequency RF generator 26, having
a controlled variable output power. As described infra, at either
the low (e.g. 4.0 MHz) or high (e.g. 13.56 MHz) frequency of
generator 26, there is a single current maximum in each of windings
40 and 42. At the high frequency, the current maxima are at
locations that are about half-way between the terminals of each
winding. The current maxima occur at radially opposite points of
the windings 40 and 42 relative to axis 44 to provide approximate
azimuthal symmetry to the ring shaped electric field resulting from
RF excitation of windings 40 and 42 by generator 26.
[0046] Windings 40 and 42 are respectively in separate parallel
circuit branches 81 and 83. Branch 81 includes series connected
winding 40 and variable capacitors 80 and 84, while branch 83
includes series connected winding 42 and variable capacitors 82 and
86. The turns of windings 40 and 42 of coil 24 are arranged so that
input terminals 46 and 50, which are driven in the parallel by
power from the output terminal of matching network 28, are on
opposite sides of coil axis 44 so current flows in the same
direction from terminals 46 and 50 into the remainder of windings
40 and 42. Terminal 46 is on the smallest radius turn 54 of coil 24
and terminal 58 is on the largest radius turn 66. Terminals 46 and
50 are respectively connected by series variable capacitors 80 and
82 to the output terminal of matching network 28.
[0047] Output terminals 48 and 52 of coil 24, which are
diametrically opposite to each other relative to axis 44, are
connected to the ground common terminals via series variable
capacitors 84 and 86.
[0048] For the high frequency output of source 26, the values of
capacitors 84 and 86 are set such that the standing wave currents
in windings 40 and 42 have minimum amplitudes at the input and
output terminals 46 and 48 of winding 40 and at terminals 50 and 52
of winding 42, where the standing wave voltages are at maxima. The
standing wave currents in windings 40 and 42 have maximum values at
radially opposite points of windings 40 and 42, were the standing
wave voltages are at minima, a result achieved by adjusting the
values of capacitors 84 and 86. The standing wave current maximum
can be located by monitoring the standing wave voltages. The
current maximum occurs at a place where the voltage is a minimum
(close to zero volt). Locating the standing wave current maxima in
windings 40 and 42 to be radially opposite to each other assists in
providing azimuthally symmetric plasma density.
[0049] The values of capacitors 80 and 82 help keep the impedance
of each of windings 40 and 42 tuned to matching network 28. The
maximum amplitudes of the standing wave currents in windings 40 and
42 are respectively controlled by the values of capacitors 80 and
82. The physical configuration of windings 40 and 42 and the
location of terminals 46, 48, 50 and 52 affect the positions of the
maximum standing wave currents in windings 40 and 42.
[0050] Proper control of the values of capacitors 80, 82, 84 and
86, as well as the total output power of generator 26, i.e., the
power that generator 26 applies in parallel to windings 40 and 42,
enables the current in one of windings 40 or 42 to remain
substantially constant, while providing changes of the current in
the other winding. The ability to vary the total power while
maintaining the current in one of windings 40 or 42 substantially
constant provides substantial control over the electromagnetic
field distribution resulting from energization of the windings. By
maintaining the current in one of windings 40 or 42 substantially
constant, the electromagnetic field produced by that winding, and
supplied to the plasma in chamber 10 remains relatively constant.
Changing the current in the other winding 40 or 42 causes the
electromagnetic field that winding supplies to the plasma in
chamber 10 to vary. As described previously, different workpiece
processing recipes require the electromagnetic power deposited by
winding 40 to remain substantially constant and the power that
winding 42 couples to the plasma to be varied. For other recipes,
it is desirable for the power distribution that winding 42 supplies
to the plasma in chamber 10 to remain constant and the power that
branch 40 supplies to the plasma in chamber 10 to be varied.
[0051] The values of capacitors 80, 82, 84 and 86, as well as the
output power of generator 26, are controlled for different recipes
by manual adjustment of these parts or by automatic adjustment
thereof in response to signals stored in memory system 35 being
read out by microprocessor 33 in response to recipe signals from
input controller 41. Alternatively, if a particular coil always
operates in connection with a processor that always operates with
the same recipe, the values of capacitors 80, 82, 84 and 86, as
well as the output power of generator 26, can be set at the
factory, at the time the processor is manufactured, or during
installation of the processor.
[0052] Assume each of windings 40 and 42 typically has a resistance
of 6 ohms, which enables the RMS (root mean squared) current in
winding 42 to be maintained substantially constant and the RMS
current in winding 40 to be varied by adjusting the output power of
generator 26 and the total reactances (X.sub.1) and (X.sub.2) of
branches 81 and 83 to be in accordance with Table I:
1TABLE 1 (R.sub.1 = R.sub.2 = 6.OMEGA.) .dwnarw. Cases P.sub.tot(W)
X.sub.1(.OMEGA.) X.sub.2(.OMEGA.) I.sub.1(A) I.sub.2(A) a) Equal
currents 1000 40 40 9.13 9.13 in 40 and 42 b) Larger current 1570
20 30 13.36 9.12 in 40 than 42 c) Lower current 850 60 50 7.63 9.14
in 40 than 42
[0053] Similarly, if it is desired to maintain a substantially
constant current in interior winding 40 and a variable current in
outer winding 42, the reactances of branch 81 (X.sub.1) and branch
82 (X.sub.2) and the output power of generator 26 are adjusted in
accordance with Table II.
2TABLE II (R.sub.1 = R.sub.2 = 6.OMEGA.) .dwnarw. Cases
P.sub.tot(W) X.sub.1(.OMEGA.) X.sub.2(.OMEGA.) I.sub.1(A)
I.sub.2(A) a) Equal currents 1000 40 40 9.13 9.13 in 40 and 42 b)
Larger current 1570 30 20 9.12 13.36 in 40 than 42 c) Lower current
850 50 60 9.14 7.63 in 40 than 42
[0054] By varying the values of capacitors 80, 82, 84 and 86, as
well as the power of source 26, control of the plasma density
incident on workpiece in both the azimuthal and radial coordinate
directions is achieved.
[0055] The following analysis of branches 81 and 82 provides a
quantitative insight into deriving appropriate values for
impedances associated with the branches.
[0056] Assume the currents and the impedances are I.sub.1, and
z.sub.1, respectively for branch 81, and are I.sub.2 and z.sub.2,
respectively for branch 83. Since each branch consists of the
series combination of an input capacitor, a winding and an output
capacitor, the impedance z.sub.1 or z.sub.2 is the lump sum of all
the impedances from the input (C.sub.1 or C.sub.2) and the output
(C.sub.3 or C.sub.4) capacitors, and the winding (L.sub.1or
L.sub.2) for branch 81 or branch 83. Thus
z.sub.1=R.sub.1+j[.omega.L.sub.1-1/(.omega.C.sub.1)=1/(.omega.C.sub.3]=.s-
ub.1+jX.sub.1, where R.sub.1 and
X.sub.1=.omega.L.sub.1-1/(.omega.C.sub.1)- -1/(.omega.C.sub.3)
respectively represent the real (resistive) and imaginary
(reactive) parts of impedance z.sub.1. Similarly, z.sub.2=R.sub.2+j
X.sub.2, where R.sub.2 and X.sub.2=.omega.L.sub.2-1(.om-
ega.C.sub.2)-1/(.omega.C.sub.4) respectively represent the
resistive and reactive parts of impedance z.sub.2. Let V be the RF
voltage across either branch; I be the total current supplied to
branches 81 and 83; P.sub.tot be the total power dissipated in the
two branches, i.e., the output power of source 26; and z be the
overall impedance of the two branches. Because branches 81 and 83
are in parallel 1 z = z 1 z 2 z 1 + z 2 = ( R 1 + j X 1 ) ( R 2 + j
X 2 ) ( R 1 + R 2 ) + j ( X 1 + X 2 ) ( 1 )
[0057] The impedance given by Equation (1) can be rewritten as
z=.vertline.z.vertline.e.sup.j.phi.=R+jX, where R is the overall
real component of coil 24, i.e., of windings 40 and 42 in parallel
with each other and is obtained from Equation (1) as: 2 R = ( R 1 +
R 2 ) ( R 1 R 2 - X 1 X 2 ) + ( X 1 + X 2 ) ( R 1 X 2 + R 2 X 1 ) (
R 1 + R 2 ) 2 + ( X 1 + X 2 ) 2 ( 2 ) = R 1 ( R 2 2 + X 2 2 ) + R 2
( R 1 2 + X 1 2 ) ( R 1 + R 2 ) 2 + ( X 1 + X 2 ) 2 ( 2 a )
[0058] From Equation 2(a), R is more sensitive to changes of
R.sub.1 and R.sub.2 than to changes of X.sub.1 and X.sub.2.
[0059] Then P.sub.tot is given by 3 P tot = 1 2 V o I o cos = 1 2 I
o 2 z cos = 1 2 I o 2 R = I r m s 2 R ( 3 )
[0060] where V.sub.o and I.sub.o are respectively peak amplitudes
of the voltage and current matching network 28 applies to coil 24,
I.sub.rms is the rms current I matching network 28 applies to coil
24, and .phi. is the phase difference between the voltage and
current matching network 28 applies to coil 24 since
V/I=z=.vertline.z.vertline.e.sup.j.phi.. Moreover
V.sub.o-I.sub.o.vertline.z.vertline. and I.sub.rms=I.sub.o/{squa-
re root}{square root over (2)}.
[0061] The current I.sub.1 in branch 81 can be calculated from
Equations (1) and (3), as 4 I 1 = V z 1 = I z z 1 = z 2 z 1 + z 2 I
( 4 )
[0062] The rms value of I.sub.1 is obtained by substituting
Equations (2) and (3) into (4) 5 I 1 ( r m s ) = z 2 z 1 + z 2 I r
m s = R 2 2 + X 2 2 ( R 1 + R 2 ) 2 + ( X 1 + X 2 ) 2 P tot R
Similarly , ( 5 ) I 2 ( r m s ) = z 1 z 1 + z 2 I r m s = R 1 2 + X
1 2 ( R 1 + R 2 ) 2 + ( X 1 + X 2 ) 2 P tot R ( 6 )
[0063] Equations (5) and (6) clearly show that the currents in
branches 81 and 83, and windings 40 and 42 are coupled. For a given
constant total power P of source 26, as X.sub.1 increases (by
decreasing the value of the input capacitor 80 in branch 81),
I.sub.1 decreases while I.sub.2 increases. Since R (from Equation
2(a)) changes very little as X.sub.1 or X.sub.2 changes, R can be
approximately treated as a constant in the discussion here.
[0064] For simplicity, assume the frequency of RF source 26 is
13.56 MHz and the electrical length of each of branches 40 and 42
is 77.degree. at 13.56 MHz and the values of capacitors 80, 82, 84
and 86 are properly adjusted so there are equal currents in
windings 40 and 42. For such a situation, the standing wave current
amplitudes along the lengths of each of windings 40 and 42 are as
depicted by curve 90, FIG. 3. Curve 90 has a sinusoidal-like
variation between the input and output terminals of each of
windings 40 and 42. Curve 90 has a peak value of approximately 14.5
amperes RMS at the midpoint of the curve, i.e., at 38.degree., and
minimum equal values of about 10.7 amperes RMS at the input and
output terminals of windings 40 and 42. Thus, the maximum standing
wave current in each of windings 40 and 42 exceeds the minimum
standing wave current by approximately 3.8 amperes RMS, i.e., by
about 21%.
[0065] A possible problem with operating the arrangement of FIG. 2
at a frequency of 13.56 MHz is that capacitors 80, 82, 84 and 86
might have to be adjusted simultaneously or in an iterative manner
to maintain the desired relationship for the electromagnetic field
distributions derived from windings 40 and 42. For example, to
maintain an azimuthally symmetric density on the workpiece,
requires the maximum currents in each coil to be located radially
opposite to each other relative to axis 44. This is achieved by
adjusting the values of capacitors 84 and 86 connected between the
output terminals of windings 40 and 42 and ground. Adjusting the
values of capacitors 84 and 86 may require adjustment of capacitors
80 and 82 to provide the desired values of standing wave current to
achieve the values indicated in Tables I and II. However, adjusting
the values of capacitors 80 and 82 can cause a further shift in the
current standing wave patterns in windings 40 and 42, whereby the
maxima of the current standing wave patterns are no longer
diametrically opposed relative to coil axis 44. If the current
standing wave maxima are shifted in this manner, further adjustment
of the values of capacitors 84 and 86 may be necessary.
[0066] To overcome this problem, we have realized that if the
current variations along windings 40 and 42 can be substantially
reduced, such that the location of the standing wave current maxima
in windings 40 and 42 is not critical, i.e., the maxima do not have
to be on diametrically opposite sides of coil axis 44, that only a
single variable capacitor need be connected to each of windings 40
and 42. In other words, the necessity to simultaneously or
iteratively adjust all four capacitors 80, 82, 84 and 86 would be
obviated.
[0067] To these ends, one embodiment of the invention involves
reducing the frequency of RF source 26 so that the transmission
line effects of windings 40 and 42 are substantially reduced. If
the electrical length of each of windings 40 and 42 is
substantially less than about 45.degree., the percent change
between the maximum and minimum values of the standing wave current
is reduced sufficiently to enable only a single variable capacitor
84 and 86 to be connected in series with windings 40 and 42,
respectively, and the need for any capacitor to be connected
between each winding input terminal and the power output terminal
of matching network 28 is obviated.
[0068] As mentioned previously, in one preferred embodiment, the
frequency of RF source 26 is reduced to 4.0 MHz from 13.56 MHz,
resulting in a decrease in the electrical length of windings 40 and
42 by a factor of 3.4. Curve 92, FIG. 3, represents the situation
of capacitors 84 and 86 being adjusted so equal standing wave
currents are in windings 40 and 42. The same physical windings that
are analyzed at 13.56 MHz (shown in curve 90 ) are re-analyzed at
4.0 MHz (shown in curve 92 ). The electrical length of each of
branches 40 and 42 becomes 22.6.degree. (i.e., 77.degree./3.4).
Curve 92 has standing wave current minima of approximately 25.7
amperes RMS at the input and output terminals of windings 40 and 42
and a maximum standing wave current of approximately 26 amperes RMS
at the centers of the windings. Despite the fact that substantially
larger current flows in windings 40 and 42 for the short
transmission line situation of curve 92 than for the long
transmission line situation of curve 90, the output power of source
24 was the same, 2400 watts, for both situations. For the exemplary
equal current curves 90 and 92 of FIG. 3, the capacitances of
capacitors 84 and 86 are equal to each other and have a value of
137 picofarads (pF) for the 13.56 MHz frequency of source 26, while
the values of capacitors 84 and 86 are 1808 pF for the 4.0 MHz
excitation of source 26.
[0069] The percentage change between the maxima and maximum
standing wave currents of curve 92 is about 2%, in contrast with
the 21% change of curve 90. Because a relatively low frequency of
excitation source 26 results in a relatively small change between
the minima and maximum standing wave currents of windings 40 and
42, there is a relatively uniform azimuthal electric field produced
by each of windings 40 and 42 to the plasma in chamber 10.
Consequently, the need to include capacitors 80 and 82, to adjust
the position of the maximum standing wave currents in windings 40
and 42 does not exist. Tables I and II provide the information
necessary for the low frequency excitation to adjust the
capacitances of capacitors 84 and 86 and the output power of RF
source 26 to achieve constant currents in coils 40 and 42,
respectively.
[0070] The ratio (I.sub.1/I.sub.2)of the maximum standing wave
currents in windings 40 and 42 can be varied continuously from 20:1
to 1:1, then from 1:1 to 1:10, for the 4.0 MHz excitation power of
source 26, by adjusting the value of capacitor 84, while
maintaining the value of capacitor 86 constant, and then by
adjusting the value of capacitor 86, while maintaining the value of
capacitor 84 constant, where I.sub.1 is the maximum standing wave
current in winding 40 and I.sub.2 is the maximum standing wave
current in winding 42. As the values of capacitors 84 and 86 are
varied, the output power of source 26 is varied to provide the same
effects as indicated by Tables I and II.
[0071] To control the values of capacitors 80, 82, 84 and 86, in
response to output signals of microprocessor 33, each of the
capacitors is driven by a different one of DC motors 87. Each of
motors 87 responds to a different output signal of microprocessor
33. The signals microprocessor 33 supplies to motors 87 have values
commensurate with the amount that the output shafts of the motors
are to be turned to achieve the desired capacitance values of
capacitors 80, 82, 84 and 86. Matching network 28 includes variable
reactances (preferably capacitors, FIG. 4) which are driven by
motors 88. Motors 88 respond to different signals microprocessor 33
derives in response to signals derived by a program stored by
memory system 35 and detector 43. Detector 43 derives signals
representing (1) the voltage amplitude reflected by matching
network 28 toward generator 26 and (2) the difference in phase
between the reflected voltage and current. Microprocessor 33
supplies a suitable DC signal to generator 26 to control the
generator output power. Microprocessor 33 responds to signals
indicative of the voltage applied in parallel to branches 81 and 83
and by RF source 26 and matching network 28, as well as signals
indicative of the standing wave currents at the output terminals 48
and 52 of branches 81 and 83, as derived by circuitry described in
connection with FIG. 4.
[0072] Reference is now made to FIG. 4 of the drawing, a circuit
diagram of a preferred embodiment of electronic circuitry
associated with 4.0 MHz drive of coil 24. RF source 26 drives
matching circuit 28 via phase and magnitude detectors 43 and fixed
series capacitor 100, preferably having a capacitance of 2000 pF.
Matching network 28 includes variable shunt capacitor 102 and
variable series capacitor 104 having capacitance values which are
varied by motors 88.
[0073] The output power of matching circuit 28 is coupled in
parallel to branches 81 and 83 via series inductor 106, RF voltage
detector 108 and phase detector 109. RF voltage detector 108
derives a DC voltage indicative of the peak amplitude of the RF
voltage at the joint input terminals of branches 81 and 83, while
phase detector 109 derives a DC voltage indicative of the
difference in phase between the RF voltage and current at the joint
input terminals of branches 81 and 83. The output voltages of
detectors 108 and 109 are fed back to microprocessor 33 which in
turn controls motors 87 and the output power of generator 26 to
achieve the previously discussed results. The currents flowing
through branches 81 and 83 are respectively coupled to ground via
variable capacitors 84 and 86.
[0074] The magnitudes of standing wave currents at output terminals
of branches 81 and 83 are respectively detected by current
amplitude sensors 110 and 112, respectively inductively coupled to
wire leads 111 and 113 that are connected between the low voltage
electrodes of capacitors 84 and 86 and ground. Each of current
sensors 110 and 112 includes a current transformer including
toroidal coil 109 having a central opening through which the wire
leads 111 and 113 extend to provide the inductive coupling. Each of
current sensors 110 and 112 also includes a rectifier and low-pass
filter for supplying to microprocessor 33 a DC current indicative
of the currents respectively flowing through terminals 48 and
52.
[0075] Grounded electromagnetic shields 114 and 116 are
respectively interposed between current sensors 110 and 112 and
capacitors 84 and 86 to minimize electromagnetic interference from
RF fields of the remaining apparatus, particularly from windings 40
and 42. Shield 114 or 116 consists of a ring-shaped metal plate 119
and shield 121. Shield 119 has an opening for lead 111 or 113 to
run through. Shield 121 is a metal cylinder which horizontally
encloses sensor 110 or 112 and lead 111 or 113. Together with
shield 119 and plate 115, which vertically sandwich the sensor,
sensor 110 and 112 and lead 111 or 113 are completely shielded from
ambient RF fields, thereby greatly improving the accuracy of the
current sensor. Shields 119 and 121 are preferably made of
silver-plated copper. Shield 121 is mechanically and electrically
connected only to plate 115. All the voltages at the output
terminals of windings 40 and 42 are across capacitors 84 and 86 so
end plates 142 of the capacitors connected to leads 111 and 113 are
virtually at ground. Shields 114 and 116 and current detectors 110
and 112 are arranged together with detector 43, capacitors 100, 102
and 104, coil 106 and detectors 108 and 109 in metal housing 117.
The details of the current sensors 110 and 112 and shields 114 and
116 are described by our co-pending application entitled "INDUCTIVE
PLASMA PROCESSOR INCLUDING CURRENT SENSOR FOR PLASMA EXCITATION
COIL" (Lowe Hauptman Gilman and Berner Docket No. 2328-051).
[0076] Each of capacitors 84, 86, 100, 102 and 104 is a vacuum
capacitor capable of handling relatively large currents which flow
from RF source 26 to windings 40 and 42. Because of the relatively
short electrical length of each of windings 40 and 42 at 4 MHz,
relatively large capacitance values are required for capacitors 84
and 86, with typical maximum values of the capacitors being 2500
pF. Shunt load capacitor 102 has a relatively large maximum value
of 1400 pF to match the low impedance of parallel branches 81 and
83. Series capacitor 104 is a relatively large capacitor, having a
maximum value of 1500 pF to tune the low inductive reactances of
parallel branches 81 and 83.
[0077] Fixed input series connected capacitor 100, preferably
having a value of 200 pF, provides part of the impedance
transformation between source 26 and the parallel windings 40 and
42 of coil 24. Capacitor 100 is included to enable shunt, load
capacitor 102 to have a more reasonable value; otherwise, capacitor
102 would have a considerably higher capacitance value than the
values associated with a capacitor having a maximum value of 1400
pF. Fixed capacitor 100 also provides better tuning resolution, to
attain better resonant tuning of matching circuit 28 with parallel
windings 40 and 42 of coil 24.
[0078] Fixed inductor 106, preferably having a relatively large
value of 3.5 microhenries, extends the tuning range of matching
network 28. Inductor 110, which is outside housing 117 and is
optionally connected to interior winding 40, can be employed to
provide substantially equal impedances for the parallel branches 81
and 83 associated with windings 40 and 42. Inductor 110 is used if
winding 42 has an inductance substantially greater than the
inductance of winding 40.
[0079] Voltage detector 108 and current sensors 110 and 112 supply
signals to microprocessor 33. Microprocessor 33 responds to the
signals from voltage detector 108, current sensors 110 and 112 and
the phase indication detector 109 and derives the total output
power RF source 26. The indication of total power is used to
control the output power of RF generator 26 to enable the powers
indicated by Tables I and II to be achieved. The signals that
current sensors 110 and 112 derive are used by microprocessor 33 to
control the motors which vary the capacitances of capacitors 84 and
86 to assure that the correct currents are flowing in windings 40
and 42 to achieve the currents specified in Tables I and II.
[0080] FIG. 5 is a contour plot of sputter rate on an 8-inch
semiconductor wafer uniformly covered by an oxide layer. The plot
resulted from an inductive discharge using windings 40 and 42 shown
in FIG. 2. Windings 40 and 42 were driven by a 1500 watt output of
generator 26, at a frequency of 4 MHz. The electrode in chuck 30
was driven by a 1400 watt output of source 45 at a frequency of 4
MHz, creating a DC bias of -375V in the chemistries of 85 sccm
(cm.sup.3/minute) of Ar and 100 sccm of O.sub.2 with a total
pressure of 5 mTorr. Capacitors 80 and 82 were omitted and
capacitors 84 and 86 were adjusted so the ratio of standing wave
maximum currents in inner winding 40 (I.sub.1) to that in outer
winding 42 (I.sub.2) is fixed at I.sub.1/(I.sub.2)=1.4:1.9. The
powers dissipated in the inner winding and in the outer winding are
roughly balanced. The sputter rate contour plot indicates a uniform
plasma density. The spatially averaged sputter rate indicated by
line 170 is 1211 angstroms/minute and standard deviation is 3.2%.
The "+" sign in FIG. 5 indicates a sputter rate higher than the
average while the "-" sign denotes a sputter rate lower than the
average. The equipment that produced the contour plot of FIG. 5
produces contours for each 50 angstroms of etched material. Since
there is only one contour line in FIG. 5, etching was within.+-.50
angstroms of average contour line 170. By changing the ratio
(I.sub.1I/.sub.2) of current in the inner and the outer windings 40
and 42 so it is higher than unity, about unity and less than unity,
the plasma density incident on the workpiece is varied radially
from dense in the center, to uniform, and to dense in the outer
edge.
[0081] When the processor is being made, interior winding 40 is
turned relative to exterior winding 42 to assist in controlling the
azimuthal electric field distribution and the azimuthal plasma
density distribution. Winding 40 is turned about axis 44 so
terminals 46 and 48 are at different positions relative to
terminals 50 and 52. In other words, terminals 46 and 48 can be at
locations different from those illustrated in FIG. 2. Winding 40
can be turned to a predetermined position if the processors of the
same type have consistent azimuthal electric field and plasma
density distributions from processor to processor. If, however,
different processors of the same type have differing azimuthal
electric field and plasma density distributions from processor to
processor, winding 40 is turned relative to winding 42 until tests
indicate optimum uniform plasma distribution is achieved in each
particular processor.
[0082] While there have been described and illustrated specific
embodiments of the invention, it will be clear that variations in
the details of the embodiments specifically illustrated and
described may be made without departing from the true spirit and
scope of the invention as defined in the appended claims. For
example, many of the principles of the invention are not limited to
coils having two concentric windings but are applicable to coils
having three or more windings.
* * * * *